Method and apparatus for controlling wireless induction power supply

ABSTRACT

The present invention provides a method and an apparatus for controlling the wireless induction power supply. The apparatus comprises a transmitter control circuit and a receiver control circuit. The method comprises generating a plurality of switching signals for switching a transmitter winding and generating a power; detecting a level of a transmitter signal from the transmitter winding; and controlling a switch to deliver the power from a receiver winding to a load. The receiver winding is coupled to receive the power from the transmitter winding. The switching signals will be disabled if the level of the transmitter signal is not higher than a threshold over a first period or the level of the transmitter signal is higher than a high-threshold over a second period. Accordingly, the method and the apparatus according to the present invention have the foreign object detection (FOD) function for the safety.

REFERENCE TO RELATED APPLICATIONS

This Application is based on Provisional Application #61/811,215, filed12 Apr. 2013, and currently pending.

FIELD OF THE INVENTION

The present invention relates to power supply, and more particularly,relates to method and apparatus for controlling the wireless inductionpower supply.

DESCRIPTION OF THE RELATED ART

The wireless induction power supply has been used to provide the powersource for electronic products and charge the battery for the electronicproducts. The danger may be happened when the foreign object, such asmetal object, is coupled to the wireless induction power supply by themagnetic field of the wireless induction power supply. Accordingly, thepresent invention provides a method and an apparatus for controlling thewireless induction power supply, which can detect the foreign object forthe safety.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a method and anapparatus for controlling the wireless induction power supply. Itincludes the foreign object detection (FOD) function for the safety. TheFOD function will prevent induction heating the foreign object.

It is an objective of the present invention to provide a method and anapparatus for controlling the wireless induction power supply. It canmake the wireless induction power supply save the power when the loadcondition of the wireless induction power supply is the no loadcondition.

The method for controlling the wireless induction power supply accordingto the present invention comprises generating a plurality of switchingsignals for switching a transmitter winding and generating a power;detecting a level of a transmitter signal from the transmitter winding;and controlling a switch to deliver the power from a receiver winding toa load. The receiver winding is coupled to receive the power from thetransmitter winding. The switching signals will be disabled if the levelof the transmitter signal is not higher than a threshold over a firstperiod or the level of the transmitter signal is higher than ahigh-threshold over a second period.

The apparatus for controlling the wireless induction power supplyaccording to the present invention comprises a transmitter controlcircuit and a receiver control circuit. The transmitter control circuitgenerates a plurality of switching signals for switching a transmitterwinding and generating a power. The transmitter control circuit furtherdetects a level of a transmitter signal from the transmitter winding.The receiver control circuit controls a switch to deliver the power froma receiver winding to a load. The receiver winding is coupled to receivethe power from the transmitter winding. The switching signals will bedisabled if the level of the transmitter signal is not higher than athreshold over a first period or the level of the transmitter signal ishigher than a high-threshold over a second period.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understandingof the invention, and are incorporated into and constitute a part ofthis specification. The drawings illustrate embodiments of the inventionand, together with the description, serve to explain the principles ofthe invention.

FIG. 1 shows a circuit diagram of an embodiment of the system for awireless induction power supply according to the present invention.

FIG. 2 shows the waveforms of the transmitter signal V_(X), the controlsignal S_(W), and the signal V_(B) according to the present invention.

FIG. 3 is a control flow chart of an embodiment of the transmittercontrol circuit controlling the switching signals according to thepresent invention.

FIG. 4 is a flow chart of an embodiment of checking the load conditionaccording to the present invention.

FIG. 5 is a flow chart of an embodiment of checking the heavy loadcondition according to the present invention.

FIG. 6 is a flow chart of an embodiment of checking the no loadcondition according to the present invention.

FIG. 7 is a control flow chart of an embodiment of the receiver controlcircuit controlling the switch according to the present invention.

FIG. 8 shows the waveforms of the switching signals S₁, S₂, the controlsignal S_(W), and the signal V_(B) according to the present invention.

FIG. 9 shows the waveforms of the switching signals S₁, S₂, the controlsignal S_(W), and the signal V_(B) during the foreign object is detectedaccording to the present invention.

FIG. 10 shows the waveforms of the switching signals S₁, S₂, and thesignal V_(B) during the no load condition or the foreign object beingdetected according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a circuit diagram of an embodiment of the system for awireless induction power supply according to the present invention. Atransmitter winding 10 is coupled between a second terminal of acapacitor 15 and a ground. A first transistor 40 is coupled between aninput terminal of the wireless induction power supply and a firstterminal of the capacitor 15. Therefore, an input voltage V_(IN) isdelivered to the transmitter winding 10 through the first transistor 40and the capacitor 15. A second transistor 45 is coupled between thefirst terminal of the capacitor 15 and the ground. Therefore, thetransistors 40 and 45 are coupled to the transmitter winding 10 throughthe capacitor 15 to switch the transmitter winding 10.

A transmitter control circuit 100 generates switching signals S₁ and S₂to drive the transistors 40 and 45 for switching the transmitter winding10 through the transistors 40, 45 and the capacitor 15. In other words,the transistors 40 and 45 develop a bridge topology for switching thetransmitter winding 10. The transmitter control circuit 100 is furthercoupled to the first terminal of the capacitor 15. The switching signalsS₁ and S₂ switch the transistors 40 and 45 to drive the transmitterwinding 10 generate a transmitter signal V_(X). The transmitter winding10 will generate a magnetic field to generate the power in response tothe input voltage V_(IN). The power is a transmitter power. Thetransmitter winding 10 will transfer the power to a receiver winding 20via a gap 30 (such as an air-gap). The receiver winding 20 will receivethe power transferred from the transmitter winding 10 and generate a DCvoltage V_(Y) at a capacitor 25 through a plurality of rectifiers 21,22, 23, and 24.

The first rectifier 21 is coupled between a first terminal of thereceiver winding 20 and a first terminal of the capacitor 25. A secondterminal of the capacitor 25 is coupled to another ground. The secondrectifier 22 is coupled between a second terminal of the receiverwinding 20 and the first terminal of the capacitor 25. The thirdrectifier 23 is coupled between the second terminal of the capacitor 25and the first terminal of the receiver winding 20. The fourth rectifier24 is coupled between the second terminal of the capacitor 25 and thesecond terminal of the receiver winding 20.

A switch 70 is connected between the first terminal of the capacitor 25and a load 80 to deliver the power (DC voltage V_(Y)) from the capacitor25 to the load 80. The load 80 includes a energy storage device such asbattery and/or a capacitor 85 for the energy storage. A receiver controlcircuit 200 is coupled to detect the DC voltage V_(Y) and a voltageV_(O) of the load 80. The voltage V_(O) is related to the DC voltageV_(Y) and the load 80. The receiver control circuit 200 will generate acontrol signal S_(W) to control the on/off of the switch 70.

The transmitter control circuit 100 is further coupled to detect thelevel of the input voltage V_(IN) via a voltage divider developed byresistors 61 and 62. The voltage divider divides the input voltageV_(IN) to generate a voltage V_(A). The voltage V_(A) is coupled to thetransmitter control circuit 100 for transmitter control circuit 100detecting the input voltage V_(IN). In one embodiment of the presentinvention, the transmitter control circuit 100 generates the switchingsignals S₁ and S₂ in response to the level of the input voltage V_(IN).

A capacitor 50, resistors 51, 52, and a diode 53 develop an attenuatorcoupled to the transmitter winding 10 for detecting the transmittersignal V_(X). The capacitor 50 is coupled between the transmitterwinding 10 and a first terminal of the resistor 51. The resistor 52 iscoupled between a second terminal of the resistor 51 and the ground. Ananode of the diode 53 is coupled to the ground and the resistor 52. Acathode of the diode 53 is coupled to a joint of the resistors 51 and52. The attenuator is utilized to attenuate the transmitter signal V_(X)for detecting the transmitter signal V_(X) easily. A resistor 56 and acapacitor 57 form a filter for generating a signal V_(B) according to anoutput of the attenuator. The signal V_(B) represents the transmittersignal V_(X). Accordingly, the attenuator and the filter are used fordetecting the transmitter signal V_(X) easily. In practical design, thetransmitter signal V_(X) can be detected by a variety of circuit.

A first terminal of the resistor 56 is coupled to the cathode of thediode 53 and the joint of the resistors 51 and 52. A first terminal ofthe capacitor 57 is coupled to a second terminal of the resistor 56 andthe transmitter control circuit 100. A second terminal of the capacitor57 is coupled to the ground.

The signal V_(B) is coupled to the transmitter control circuit 100. Thelevel of the signal V_(B) is correlated to the level of the transmittersignal V_(X). Therefore, the transmitter control circuit 100 can detectthe level of the transmitter signal V_(X) by detecting the level of thesignal V_(B). The level of the transmitter signal V_(X) is related tothe level of the input voltage V_(IN) and the impedance (load) of thetransmitter winding 10. The impedance of the transmitter winding 10 isdetermined by the object coupled to the transmitter winding 10 by themagnetic field of the transmitter winding 10. A lower impedance of thetransmitter winding 10 will produce a lower level of the transmittersignal V_(X).

The impedance of the transmitter winding 10 is related to the impedanceof the receiver winding 20 according to the embodiment. The impedanceacross to the receiver winding 20 will be propositional to the impedanceof the transmitter winding 10. For example, both the impedance acrossthe receiver winding 20 and the transmitter winding 10 will become lowerwhen the switch 70 is turned on and the load 80 is coupled to thereceiver winding 20. The impedance of the transmitter winding 10 is alsorelated to the impedance of the foreign object that is coupled to thetransmitter winding 10 by the magnetic field of the transmitter winding10. For example, a foreign object (material) with the characteristic ofhigh eddy-current will cause low impedance to the transmitter winding10.

FIG. 2 shows the waveforms of the transmitter signal V_(X), the controlsignal S_(W), and the signal V_(B) according to the present invention.The level of the signals V_(X) and V_(B) will be decreased in responseto the turn on of the switch 70 (control signal S_(W)) shown in FIG. 1.FIG. 3 is a control flow chart of an embodiment of the transmittercontrol circuit 100 controlling the switching signals S₁ and S₂according to the present invention. The transmitter control circuit 100(as shown in FIG. 1) will start the operation shown in the block 110.According to the block 110, the transmitter control circuit 100 willenable the switching signals S₁ and S₂ (transfer the power from thetransmitter winding 10 to the receiver winding 20, as shown in FIG. 1)and reset a variable N and a variable M to zero. The variable Nrepresents a period of a heavy load condition. The variable M representsa period of a no load condition. Then, according to the block 150, thetransmitter control circuit 100 will check the load condition and updatethe variables N and M according to the load condition.

According to the block 151, the transmitter control circuit 100 willcheck whether the value of the variable N is higher than a constant A ornot. If the value of the variable N is higher than the constant A, thenthe switching signals S₁ and S₂ will be disabled (stop the powertransfer of the transmitter winding 10) by the transmitter controlcircuit 100 according to the block 153. After that, the transmittercontrol circuit 100 will delay the disable (keep the disable) of theswitching signals S₁ and S₂ for a first delay time T_(A) (disableperiod) according to the block 155, and then the transmitter controlcircuit 100 repeats the operation of the block 110, which the controlflow returns to the block 110. The first delay time T_(A) shown in theblock 155 is a constant.

According to the blocks 151 and 157, if the value of the variable N isequal to or lower than the constant A, then the transmitter controlcircuit 100 will check whether the value of the variable M is higherthan a constant B or not. If the value of the variable M is higher thanthe constant B, then the switching signals S₁ and S₂ will be disabled bythe transmitter control circuit 100 according to the block 158. Afterthat, the transmitter control circuit 100 will delay the disable (keepthe disable) of the switching signals S₁ and S₂ for a second delay timeT_(B) (disable period) according to the block 159, and then thetransmitter control circuit 100 repeats the operation of the block 110,which the control flow returns to the block 110. The second delay timeT_(B) shown in the block 159 is a constant. According to the block 157,if the value of the variable M is equal to or lower than the constant B,then the transmitter control circuit 100 will repeats the operation ofthe block 150, which the control flow returns to the block 150.

Therefore, if the heavy load condition is continuing over a first period(N>A), then the switching signals S₁ and S₂ will be disabled for thefirst delay time T_(A) (disable period) to limit the power deliveredfrom the transmitter winding 10 to the foreign object coupled to thetransmitter winding 10 by the magnetic field of the transmitter winding10. The constant A represents the first period that is an over-loadperiod. If the no load condition is continuing over a second period(M>B), then the switching signals S₁ and S₂ will be disabled for thesecond delay time T_(B) (disable period) to save the power. The constantB represents the second period that is a no-load period. When theforeign object is detected or the no load condition is detected, thetransmitter control circuit 100 disables the switching signals S₁ and S₂and will start a status polling operation (shown in FIG. 10), which thetransmitter control circuit 100 will periodically enable the switchingsignals S₁ and S₂, and then the level of the signal V_(B) couldincrease. Therefore, the transmitter control circuit 100 detects thelevel of the signal V_(B) for detecting the transmitter signal V_(X) (asshown in FIG. 1).

FIG. 4 is a flow chart of an embodiment of checking the load conditionaccording to the present invention. According to the blocks 160 and 170,the operation of checking the load condition includes checking the heavyload condition and the no load condition. The transmitter controlcircuit 100 (as shown in FIG. 1) further detects the foreign objectaccording to the block 160, which the transmitter control circuit 100does the foreign object detection (FOD).

FIG. 5 is a flow chart of an embodiment of checking the heavy loadcondition according to the present invention. According to the block161, the transmitter control circuit 100 (as shown in FIG. 1) will checkwhether the level of the signal V_(B) is lower than a threshold V_(TL)or not. That is, the transmitter control circuit 100 checks whether thelevel of the transmitter signal V_(X) (as shown in FIG. 1) is lower thana low-threshold or not. The low-threshold is correlated to the thresholdV_(TL). If the level of the signal V_(B) is lower than the thresholdV_(TL), that the level of the transmitter signal V_(X) is lower than thelow-threshold, the load condition is the heavy load condition. Accordingto the blocks 161 and 163, if the level of the signal V_(B) is lowerthan the threshold V_(TL), then the transmitter control circuit 100 setsvariable N=N+1.

According to the blocks 161 and 165, if the level of the signal V_(B) isequal to or higher than the threshold V_(TL), then the transmittercontrol circuit 100 will further verify whether the level of the signalV_(B) is higher than a threshold V_(TH) or not. That is, the transmittercontrol circuit 100 will check whether the level of the transmittersignal V_(X) is higher than a high-threshold or not. The high-thresholdis correlated to the threshold V_(TH). If the level of the signal V_(B)is higher than the threshold V_(TH), then the transmitter controlcircuit 100 sets variable N=0 according to the block 167. If the levelof the signal V_(B) is equal to or lower than the threshold V_(TH), thenthe transmitter control circuit 100 will set variable N=N−1 or setvariable N=0 if variable N is equal to or less than 0 according to theblock 169.

FIG. 6 is a flow chart of an embodiment of checking no load conditionaccording to the present invention. According to the block 171, thetransmitter control circuit 100 (as shown in FIG. 1) will check whetherthe level of the signal V_(B) is higher than the threshold V_(TH) ornot. If the level of the signal V_(B) is higher than the thresholdV_(TH), that the level of the transmitter signal V_(X) is higher thanthe high-threshold, the load condition is the no load condition.According to the blocks 171 and 173, if the level of the signal V_(B) ishigher than the threshold V_(TH), then the transmitter control circuit100 sets variable M=M+1.

According to the blocks 171 and 175, if the level of the signal V_(B) isequal to or lower than the threshold V_(TH), then the transmittercontrol circuit 100 will further verify whether the level of the signalV_(B) is lower than the threshold V_(TL) or not. That is, thetransmitter control circuit 100 will check whether the level of thetransmitter signal V_(X) is lower than the low-threshold or not. If thelevel of the signal V_(B) is lower than the threshold V_(TL), then thetransmitter control circuit 100 will set variable M=0 according to theblock 177. If the level of the signal V_(B) is equal to or higher thanthe threshold V_(TC), then the transmitter control circuit 100 will setvariable M=M−1 or set variable M=0 if M is equal to or less than 0according to the block 179.

FIG. 7 is a control flow chart of an embodiment of the receiver controlcircuit controlling the switch (control signal S_(W)) according to thepresent invention. The receiver control circuit 200 (as shown in FIG. 1)will check the load condition of the load 80 (as shown in FIG. 1)according to the block 210. That is, the receiver control circuit 200will check whether the load condition of the load 80 is the no loadcondition or not. According to the blocks 210 and 211, if the loadcondition of the load 80 is the no load condition and/or the level ofthe voltage V_(O) (as shown in FIG. 1) is high, then the receivercontrol circuit 200 will disable the control signal S_(W) (as shown inFIG. 1), turn off the switch 70 (as shown in FIG. 1) and repeat theoperation of the block 210, that the control flow returns to the block210. Otherwise, the receiver control circuit 200 will check whether thelevel of the DC voltage V_(Y) is higher than a threshold V_(T1) or notaccording to the block 212. If the level of the DC voltage V_(Y) isequal to or lower than the threshold V_(T1), then the receiver controlcircuit 200 will repeats the operation of the block 210.

According to the blocks 212 and 215, if the level of the DC voltageV_(Y) is higher than the threshold V_(T1), then the receiver controlcircuit 200 will enable the control signal S_(W) to turn on the switch70 and set X=X+1 (wherein X is a variable). After that, according to theblock 220, the receiver control circuit 200 will further check whetherthe load condition of the load 80 is the no load condition or not. Ifthe load condition of the load 80 is the no load condition and/or thelevel of the voltage V_(O) is high, then the receiver control circuit200 will disable the control signal S_(W) to turn off the switch 70according to the block 211, and then repeat the operation of the block210. According to the blocks 220 and 230, if the load condition of theload 80 is not the no load condition and/or the level of the voltageV_(O) is low, then the receiver control circuit 200 will check whetherthe value of the variable X is higher than a constant K or not. If thevalue of the variable X is equal to or lower than the constant K, thenthe receiver control circuit 200 will repeat the operation of the block215. Otherwise, the receiver control circuit 200 will disable thecontrol signal S_(W) to turn off the switch 70 according to the block240, and delay the disable (keep the disable) of the control signalS_(W) for a third delay time T_(C) (disable period) according to theblock 250. The third delay time T_(C) is a constant. After that, thereceiver control circuit 200 will set variable X=0 according to theblock 260, and then repeat the operation of the block 210, that thecontrol flow returns to the block 210.

Therefore, the receiver control circuit 200 will check whether the loadcondition of the load 80 is the no load condition or not. The receivercontrol circuit 200 will turn off the switch 70 if the load condition ofthe load 80 is the no load condition. If the load condition of the load80 is not the no load condition, then the receiver control circuit 200will check whether the induction power has been inputted or not (bychecking the level of the DC voltage V_(Y)). If the load condition ofthe load 80 is not the no load condition and the induction power hasbeen inputted, then the control signal S_(W) will be enabled and theswitch 70 will be turned on to receive the induction power. Once theswitch 70 is turned on, the receiver control circuit 200 willperiodically turn off the switch 70, which will build the communication(synchronization) with the transmitter control circuit 100. The on timeof the switch 70 is determined by the variable X and constant K (X>K),and the off time of the switch 70 is decided by the third delay timeT_(C).

FIG. 8 shows the waveforms of the switching signals S₁, S₂, the controlsignal S_(W), and the signal V_(B) according to the present invention.The control signal S_(W) will be periodically disabled to periodicallyturn off the switch 70 shown in FIG. 1. The level of the signal V_(B)will be increased in response to the turn off of the switch 70 (thedisable of the control signal S_(W)). Therefore, the transmitter controlcircuit 100 (as shown in FIG. 1) can detect the synchronization bydetecting the signal V_(B).

FIG. 9 shows the waveforms of the switching signals S₁, S₂, the controlsignal S_(W), and the signal V_(B) during the foreign object is detectedaccording to the present invention. Because the foreign object willconsume the power generated from the transmitter winding 10 (as shown inFIG. 1), the level of the signal V_(B) can not be higher than thethreshold V_(TH), even the switch 70 (the control signal S_(W)) shown inFIG. 1 is turned off. Thus, the switching signals S₁ and S₂ will bedisabled to limit the power deliver to the foreign object for thesafety.

FIG. 10 shows the waveforms of the switching signals S₁, S₂, and thesignal V_(B) during the no load condition or the foreign object beingdetected according to the present invention, in which the transmittercontrol circuit 100 (as shown in FIG. 1) will do the status pollingduring the no load condition or if the foreign object is detected. Thetransmitter control circuit 100 will periodically enable the switchingsignals S₁ and S₂, and then the level of the signal V_(B) couldincrease. Therefore, the transmitter control circuit 100 can detect thelevel of the signal V_(B) for detecting the transmitter signal V_(X) (asshown in FIG. 1). The T_(M) represents the enable time of the switchingsignals S₁ and S₂. The T_(X) represents the disable time of theswitching signals S₁ and S₂.

Although the present invention and the advantages thereof have beendescribed in detail, it should be understood that various changes,substitutions, and alternations can be made therein without departingfrom the spirit and scope of the invention as defined by the appendedclaims. That is, the discussion included in this invention is intendedto serve as a basic description. It should be understood that thespecific discussion may not explicitly describe all embodimentspossible; many alternatives are implicit. The generic nature of theinvention may not fully explained and may not explicitly show that howeach feature or element can actually be representative of a broaderfunction or of a great variety of alternative or equivalent elements.Again, these are implicitly included in this disclosure. Neither thedescription nor the terminology is intended to limit the scope of theclaims.

What is claimed is:
 1. A method for controlling a wireless inductionpower supply, comprising: generating a plurality of switching signalsfor switching a transmitter winding and generating a power; detecting alevel of a transmitter signal from the transmitter winding while theswitching signals are enabled and while the power is not delivered to aload for detecting a foreign object, the switching signals remainenabled if the level of the transmitter signal is higher than athreshold, the switching signals are disabled if the level of thetransmitter signal is not higher than the threshold for a first timeperiod, and the status of the load is detected as no-load and theswitching signals are disabled if the level of the transmitter signal ishigher than a high-threshold for a second time period, wherein thehigh-threshold is greater than the threshold; and controlling a switchto deliver the power from a receiver winding to the load wherein thereceiver winding is coupled to receive the power from the transmitterwinding.
 2. The method as claimed in claim 1, wherein if the switchingsignals are disabled in response to the level of the transmitter signalis not higher than the threshold for the first period, periodicallyre-enabling the switching signals to detect the level of the transmittersignal.
 3. The method as claimed in claim 1, wherein the switchingsignals are coupled to drive a plurality of transistors coupled to thetransmitter winding; the transistors develop a bridge topology to switchthe transmitter winding.
 4. The method as claimed in claim 1, wherein ifthe switching signals are disabled in response to the level of thetransmitter signal is higher than the high-threshold for the secondperiod, re-enabling the switching signals periodically to detect thelevel of the transmitter signal.
 5. The method as claimed in claim 1,further comprising detecting a level of an input voltage of the wirelessinduction power supply.
 6. The method as claimed in claim 1, includingperiodically turning-off the switch to decouple the load from receivingthe power wherein the switch is coupled in series between the receiverwinding and the load.
 7. The method as claimed in claim 6, includingturning-off the switch when the load condition is a no load condition.8. The method as claimed in claim 1, wherein the level of thetransmitter signal is related to a level of an input voltage of thewireless induction power supply and an impedance of the transmitterwinding, the impedance of the transmitter winding becomes lower and thelevel of the transmitter signal is decreased when the foreign object iscoupled to the transmitter winding by a magnetic field of thetransmitter winding.
 9. The method of claim 1 including configuring thecontrol circuit to control the transmitter winding without receiving asignal from another controller coupled to control the receiver winding.10. A method for controlling a wireless induction power supply,comprising: generating a plurality of switching signals for switching atransmitter winding and generating a power; detecting a level of atransmitter signal from the transmitter winding while the switchingsignals remain enabled and while not delivering the power to a load fordetecting a foreign object, wherein a status of the load is detected asno foreign object is detected and the switching signals continue to beenabled if the level of the transmitter signal is higher than alow-threshold, and the status of the load is detected as no-load and theswitching signals are disabled if the level of the transmitter signal ishigher than a high-threshold for a first time period; and controlling aswitch to deliver the power from a receiver winding to the load whereinthe receiver winding is coupled to receive the power from thetransmitter winding.
 11. The method as claimed in claim 10, wherein ifthe switching signals are disabled in response to the level of thetransmitter signal is higher than the high-threshold for the first timeperiod, re-enabling the switching signals periodically to detect thelevel of the transmitter signal.
 12. The method as claimed in claim 10,wherein the switching signals are coupled to drive a plurality oftransistors coupled to the transmitter winding; the plurality oftransistors develop a bridge topology to switch the transmitter winding.13. The method as claimed in claim 10, wherein the switching signals aredisabled if the level of the transmitter signal is not higher than thelow-threshold for a second time period.
 14. The method as claimed inclaim 13, wherein if the switching signals are disabled in response tothe level of the transmitter signal is not higher than the low-thresholdfor the second time period, re-enabling the switching signalsperiodically to detect the level of the transmitter signal.
 15. Themethod as claimed in claim 10, further comprising detecting a level ofan input voltage of the wireless induction power supply.
 16. The methodas claimed in claim 10, wherein the switch is periodically turned off todecouple the load from receiving the power.
 17. The method as claimed inclaim 10, wherein the level of the transmitter signal is related to alevel of an input voltage of the wireless induction power supply and animpedance of the transmitter winding, the impedance of the transmitterwinding becomes lower and the level of the transmitter signal isdecreased when the foreign object is coupled to the transmitter windingby a magnetic field of the transmitter winding.
 18. The method asclaimed in claim 10, wherein the switch is turned off when the conditionof the load is a no load condition.
 19. An apparatus for controlling awireless induction power supply, comprising: a transmitter controlcircuit generating a plurality of switching signals for switching atransmitter winding and generating a power, and detecting a level of atransmitter signal from the transmitter winding while the switchingsignals remain enabled and while not delivering the power to a load fordetecting a foreign object, wherein the foreign object is detected andthe switching signals are disabled if the level of the transmittersignal is not higher than a threshold for a first time period, and thestatus of the load is detected as no-load and the switching signals aredisabled if the level of the transmitter signal is higher than ahigh-threshold for a second time period; and a receiver control circuitcontrolling a switch to deliver the power from a receiver winding fromto the load wherein the receiver winding is coupled to receive the powerfrom the transmitter winding.
 20. The apparatus of claim 19 wherein thetransmitter control circuit is configured to disable the switchingsignals in response to the level of the transmitter signal is less thana low threshold indicating a heavy load condition wherein the lowthreshold is less than the first threshold and less than thehigh-threshold.